The MRI apparatus is a medical diagnostic imaging apparatus which produces magnetic resonance in a nuclear spin within any slice passing transversely across an examination target, and obtains a tomographic image within the slice based on nuclear magnetic resonance signals being generated. When a high frequency coil (RF coil) irradiates a test subject placed in a static magnetic field with a high frequency magnetic field, while applying a gradient magnetic field, a nuclear spin within the test subject, for example, a nuclear spin of a hydrogen atom is excited. When the excited nuclear spin returns to the equilibrium state, a circularly polarized wave field is generated as a nuclear magnetic resonance signal. The RF coil detects this signal, and then, the signal is subjected to signal processing to create an image representing a distribution of atomic nuclei of hydrogen within a living body.
The RF coil used for the irradiation of the high frequency magnetic field is required to have a homogeneous distribution of irradiation strength and a high irradiation efficiency. It is desirable that the distribution of irradiation strength shows 70% or higher irradiation strength in an imaging region, with respect to a maximum value of the irradiation strength within the region. This is because, if the distribution of the irradiation strength is imhomogeneous, a difference may occur in the excited state of the nuclear spins depending on portions within the test subject, and this causes contrast unevenness and/or artifacts in the image being obtained.
Generally, for a cylindrical (tunnel type) MRI apparatus, there is known a cylindrical RF coil such as a birdcage coil (e.g., see Patent Document 1) and a TEM coil (e.g., see Non Patent Document 1), as a coil having a homogeneous distribution of irradiation strength, among the RF coils used for irradiation of the high frequency magnetic field.
As a method for enhancing the irradiation efficiency, there is a quadrature phase detection (QD: Quadrature Detection) method (e.g., see Patent Document 2, Non Patent Document 2, and Non Patent Document 3). The QD method uses two RF coils which carry out irradiation of the high frequency magnetic fields being orthogonal to each other, and irradiation of the high frequency magnetic fields is performed in such a manner that a phase difference in time phases of the high frequency magnetic field irradiation from the respective RF coils becomes 90 degrees. The QD method allows the circularly polarized wave field for exciting the nuclear spin of the hydrogen atom to be irradiated with a high degree of efficiency, and therefore, the irradiation strength can be enhanced theoretically by √2, compared to the case of irradiation by one RF coil. If it is converted into irradiated power, a half of the power is required only, and therefore the efficiency of the irradiated power is doubled. When the birdcage coil or the TEM coil (hereinafter, referred to as a cylindrical RF coil) is employed, two feeding ports used for the irradiation are arranged at the positions orthogonal to each other, thereby enabling irradiation of the high frequency magnetic fields according to the QD method, just by one coil.
Since the tunnel type MRI apparatus using the cylindrical RE coil is small in diameter and the length of the tunnel is long, a large person or a claustrophobic person is likely to feel more stress. In order to solve this problem, an MRI apparatus with wide examination space is required, excelling in a sense of openness with a large diameter and a short tunnel. Recently, in some cases, there are installed inside the MRI apparatus, a contrast medium injector and a nonmagnetic therapeutic instrument, so as to conduct a detailed diagnosis and treatment. Therefore, in order to reserve the space for installing various equipment to be placed in proximity to the test subject, it is requested to provide an MRI apparatus with wide examination space.
The tunnel type MRI apparatus has a structure arranging a static magnetic field magnet, a gradient magnetic field coil, an RF shield, and an RF coil, in this order from the outer side toward the inner side of the tunnel. The space inside the RF coil corresponds to the examination space for placing the test subject. Therefore, in order to expand the examination space accommodating the test subject, it seems sufficient just to increase the inner diameter of the static magnetic field magnet positioned at the outermost. However, size-up of the inner diameter of the tunnel-type static magnetic field magnet may cause a significant increase of production cost.
Generally, there is provided a distance approximately from 10 mm to 40 mm, between the RF shield and the RF coil. It is possible to consider that the examination space is made larger by reducing this distance, for instance. However, if the RF shield is placed closer to the RF coil, the RF shield current flowing on the RF shield is increased so as to cancel the high frequency magnetic field, and eventually, the irradiation efficiency of the high frequency magnetic field is significantly reduced.
Therefore, it is also conceivable to partially remove the coil conductor of the cylindrical RF coil, or to provide space between multiple RF coils, thereby partially expanding the examination space. As an example to expand the examination space by removing a part of the coil conductor, there is suggested a structure to remove a part of the conductor at opposed portions of a birdcage coil, and two semicylindrical birdcage coils are arranged in opposed manner (e.g., see Non Patent Document 4). In addition, as an example to expand the examination space by arranging two RF coils in opposed manner with space therebetween, there is suggested a partial antenna made up of a shield and a planar conductor structure (e.g., see Patent Document 3).